nope my returns the number of declared variables, see the update in the OP.

Sorry, but you are mistaken.

$ perl -wE'say(scalar(my ($x, $y)))'
Use of uninitialized value $y in say at -e line 1.
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It doesn't return the count. Like the error message shows, it returns the last element it would have returned (as I had guessed).

which unfortunately results in tst(state $x, state $y); in my case!

Or

&tst(state ($x, $y));
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But I prefer what you had over using "&".